Light wavelength converting module

ABSTRACT

A light wavelength converting module is provided in which generation of noise due to return light is prevented and a wave whose wavelength is converted can be obtained stably. The light wavelength converting module is formed by a semiconductor laser from which a fundamental wave exits, and a light wavelength converting element which is optically coupled to the semiconductor laser and which converts a wavelength of the fundamental wave which enters from the semiconductor laser. A wavelength plate is disposed at a light exiting side of the light wavelength converting element. An IR cutting filter, which serves as a removing means for removing the fundamental wave from a second harmonic, is disposed between the wavelength plate and the light wavelength converting element.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light wavelength convertingmodule, and in particular, to a light wavelength converting modulewhich, by using a light wavelength converting element, converts thewavelength of a fundamental wave which is emitted from a semiconductorlaser.

[0003] 2. Description of the Related Art

[0004] Conventionally, in a case in which a semiconductor laser isoptically coupled to a light wavelength converting element, and thewave, which exits from the light wavelength converting element and whosewavelength has been converted such as a second harmonic, is used asrecording light of a light scanning/recording device such as a laserprinter, in order to make the wave whose wavelength has been convertedmatch a scanning optical system provided at the light scanning/recordingdevice, the plane of polarization of the wave whose wavelength has beenconverted must be rotated by 90 °.

[0005] In order to rotate by 90° the plane of polarization of the wavewhose wavelength has been converted, a half-wave plate, which imparts anoptical path difference of ½ of the wavelength to the two polarizedlight components which are orthogonal, is disposed at the exiting endsurface side of the light wavelength converting element. The half-waveplate is formed from a white mica plate, a quartz crystal plate or thelike which is birefringent. When the orientation of the long axis of theelliptically polarized light which is incident on the half-wave plate isθ from the main axis of the wavelength plate, the orientation of thelong axis of the elliptically polarized light which exits from thehalf-wave plate is −θ. For example, if θ=45°, the elliptically polarizedlight incident on the half-wave plate and the elliptically polarizedlight exiting therefrom are orthogonal to one another. Namely, althoughthe elliptical shapes of the polarized lights are the same, thedirections of the long axes thereof are orthogonal to each other, andthe directions of rotation of the polarized lights are inverted.

[0006] However, the wave whose wavelength has been converted and thefundamental wave whose wavelength has not been converted both exit fromthe exiting end surface of the light wavelength converting element.Further, the half-wave plate is disposed substantially orthogonal to theoptical axis in order for the performances thereof to be exhibited asmuch as possible. When the half-wave plate is disposed at the exitingend surface side of the light wavelength converting element, a problemarises in that the fundamental wave is reflected by the half-wave plate,becomes so-called return light, again enters into the semiconductorlaser, and becomes a source of noise.

[0007] Further, tilting the half-wave plate with respect to the opticalaxis such that the light reflected at the half-wave plate does notbecome return light, has been thought of in order to overcome theaforementioned problem. However, when the half-wave plate is inclinedwith respect to the optical axis, the performances of the half-waveplate cannot be sufficiently exhibited.

SUMMARY OF THE INVENTION

[0008] The present invention was developed in light of theaforementioned, and an object of the present invention is to provide alight wavelength converting module which is formed to include a lightwavelength converting element and a semiconductor laser which isoptically coupled to the light wavelength converting element, whereinwhen a wavelength plate, which imparts a predetermined optical pathdifference to the orthogonal two polarized light components of the wavewhose wavelength is converted, is provided, the generation of noise dueto return light is prevented, and a wave whose wavelength is convertedcan be stably obtained.

[0009] In order to achieve the above object, a first aspect of thepresent invention is a light wavelength converting module comprising: asemiconductor laser from which a fundamental wave exits; a lightwavelength converting element which is optically coupled to thesemiconductor laser, and which converts a wavelength of the fundamentalwave which has entered from the semiconductor laser; a wavelength platedisposed at a light exiting side of the light wavelength convertingelement; and removing means, disposed between the wavelength plate andthe light wavelength converting element, for removing the fundamentalwave from light incident on the removing means.

[0010] In the first aspect of the present invention, the fundamentalwave which exits from the semiconductor laser enters into the lightwavelength converting element which is optically coupled to thesemiconductor laser, and is wavelength converted by the light wavelengthconverting element. A wavelength plate is disposed at the light exitingside of the light wavelength converting element. The fundamental wave isremoved from the light which exits from the light wavelength convertingelement, by a removing means which is disposed between the wavelengthplate and the light wavelength converting element, and the light fromwhich the fundamental wave is removed is incident on the wavelengthplate. In this way, by providing the removing means, which removes thefundamental wave from the light incident thereon, between the wavelengthplate and the light wavelength converting element, the fundamental wavecan be prevented from being reflected by the wavelength plate andbecoming return light. In this way, a wave whose wavelength has beenconverted can be obtained stably, without noise being generated at thesemiconductor laser.

[0011] In the above-described light wavelength converting module, theremoving means can be formed by an IR cutting filter. By using an IRcutting filter as the removing means, the fundamental wave, which isinfrared light, can be removed.

[0012] In the above-described light wavelength converting module, it ispreferable that the light wavelength converting element is directlyjoined to the semiconductor laser. By directly joining the lightwavelength converting element to the semiconductor laser, the device canbe made compact.

[0013] In the above-described light wavelength converting module, ahalf-wave plate or a quarter-wave plate, with respect to a wave whosewavelength is converted, can be used as the wavelength plate. In a casein which a half-wave plate with respect to the wave whose wavelength isconverted is used as the wavelength plate, the plane of polarization ofthe incident light can be rotated by 90°. In a case in which awavelength plate of ¼ of the wavelength with respect to the wave whosewavelength is converted is used as the wavelength plate, the incidentlight which is linearly polarized light can be changed into circularlypolarized light.

[0014] In the above-described light wavelength converting module, thewavelength plate can be disposed substantially orthogonal to an opticalaxis. In order to effectively exhibit the performances of the wavelengthplate, the wavelength plate is disposed substantially orthogonal to theoptical axis, and preferably, so as to form an angle of 0.5° or lesswith a plane which is orthogonal to the optical axis. If the wavelengthplate is tilted any more than that with respect to the optical axis, theextinction ratio of the light wavelength converting module deteriorates,which is not preferable.

[0015] In the above-described light wavelength converting module, a beamsplitter can be provided at a light exiting side of the wavelengthplate.

[0016] In particular, a structure is preferable in which a beam splitterand a photodiode are disposed at a light exiting side of the wavelengthplate, and the beam splitter and the photodiode are shielded from light.The beam splitter and the photodiode are disposed at the light exitingside of the wavelength plate and are shielded from light, so thatscattered light is not incident on the photodiode.

[0017] In the above-described light wavelength converting module, alight attenuating mechanism which attenuates light passing therethroughcan be provided at a light exiting side of the light wavelengthconverting element. When the driving current of the semiconductor laserfalls within a predetermined range, a stable output without dispersioncan be obtained. Thus, the light attenuating mechanism is provided atthe light exiting side of the light wavelength converting element. Thelight passing therethrough is attenuated in accordance with the desiredoutput light amount such that the value of the driving current of thesemiconductor laser falls within a predetermined range, and stableoutput can be obtained.

[0018] The light attenuating mechanism is preferably provided at a lightexiting side of the light wavelength converting element and at a lightentering side of the beam splitter. In a case in which the lightattenuating mechanism is provided at the light entering side of the beamsplitter, it suffices to provide the light attenuating mechanism at oneplace. Further, even in cases in which a beam splitter and a photodiodeare disposed at the light exiting side of the wavelength plate and theoutput light is monitored, dispersion in the value of the monitorcurrent can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a plan view of a light wavelength converting module ofan embodiment of the present invention.

[0020]FIG. 2 is a sectional view, taken along an optical axis, of thelight wavelength converting module of the embodiment.

[0021]FIG. 3 is a diagram for explaining wiring of the light wavelengthconverting module of the embodiment.

[0022]FIG. 4 is a circuit diagram showing a driving circuit of the lightwavelength converting module of the embodiment.

[0023]FIG. 5 is a plan view of a light wavelength converting module of asecond embodiment.

[0024]FIG. 6 is a graph showing an electric current vs. light outputcharacteristic (IL characteristic) of a wavelength stabilized laser.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] Hereinafter, an embodiment of a light wavelength convertingmodule to which the present invention is applied will be described indetail with reference to the drawings.

[0026] (First Embodiment)

[0027] As shown in FIG. 1, the light wavelength converting modulerelating to the present embodiment includes a semiconductor laser 10which has an oscillation wavelength in the infrared region, and whichincludes a first end surface (rearward exiting end surface) and a secondend surface (forward exiting end surface) which opposes the first endsurface; a mirror 12 which serves as a reflecting member and which,together with the forward exiting end surface of the semiconductor laser10, forms an external resonator; and a waveguide-type light wavelengthconverting element 14 which converts the wavelength of the fundamentalwave emitted from the semiconductor laser 10 and outputs a secondharmonic.

[0028] The semiconductor laser (LD) 10 is held by a mount 16. The lightwavelength converting element 14, which is formed by a second harmonicgenerating element (SHG), is held by a mount 18. In the state in whichthe semiconductor laser 10 and the light wavelength converting element14 are held by the respective mounts, the exit portion of thesemiconductor laser 10 and the waveguide portion (entrance portion) ofthe light wavelength converting element 14 are positioned so as tocoincide with one another, so as to form an LD-SHG unit 20. The LD-SHGunit 20 is fixed on a base plate 22. The light wavelength convertingelement 14 is thereby directly joined to the forward exiting end surfaceof the semiconductor laser 10.

[0029] The semiconductor laser 10 is a regular semiconductor laser(laser diode) having a Fabry-Perot type (FP type) unimodal spatial mode(transverse single mode). LR (low reflectance) coatings 24A, 24B withrespect to light of the oscillation wavelength, are provided at the bothend surfaces (cleavage surfaces) of the semiconductor laser 10. Forexample, the reflectance of the LR coating 24A with respect to thefundamental wave is 30%, and the reflectance of the LR coating 24B withrespect to the fundamental wave is 30%.

[0030] The light wavelength converting element 14 has a substrate 26which is formed from a crystal in which LiNbO₃, which is a ferroelectrichaving a non-linear optical effect, is, for example, 5 mol % doped withMgO (hereinafter, this structure will be abbreviated as MgO-LN). Aperiodic domain inverted structure and a channel light waveguide 30 areformed at the substrate 26. At the periodic domain inverted structure,domain inverted portions 28, at which the orientation of spontaneouspolarization parallel to the Z axis is inverted, is formed at apredetermined period A which will be described later. The channel lightwaveguide 30 extends along the periodic domain inverted structure.Further, an AR (transmissive) coating 32A with respect to thefundamental wave is formed at the semiconductor laser side end surfaceof the light wavelength converting element 14, and an AR coating 32Bwith respect to the second harmonic and the fundamental wave is formedat the exiting side end surface of the light wavelength convertingelement 14. The method of fabricating the waveguide type lightwavelength converting element 14 having a periodic domain invertedstructure is disclosed in detail in Japanese Patent ApplicationLaid-Open (JP-A) No. 10-254001.

[0031] The forward exiting end surface of the light wavelengthconverting element 14 is polished at an incline, and an inclined surfaceis formed thereat. The inclined surface is inclined, with respect to aplane orthogonal to the direction in which the channel light waveguide30 extends, at at least an angle θ (3°≦θ) in the direction in which thechannel light waveguide 30 extends. Due to the forward exiting endsurface, including the light waveguide end surface, being polished at anincline in this way, the fundamental wave is prevented from enteringagain into the channel waveguide 30, and the amount of return lightwhich returns to the semiconductor laser 10 is decreased. Note that theforward exiting end surface of the light wavelength converting element14 may be polished orthogonally with respect to the optical axis.

[0032] A collimator lens 36, which makes parallel a laser beam (backwardexiting light) 34R which exits from the rearward exiting end surface ofthe semiconductor laser 10 in a state of being scattered light, ismounted to the LD-SHG unit 20. The LD-SHG unit 20 and the collimatorlens 36 are sealed airtight, together with dry air or an inert gas suchas dry nitrogen, in a package 38 which serves as an airtight sealingmember, and are fixed within the package 38. Any of a distributedrefractive index rod lens such as a SELFOC lens (trade name), anaspherical lens, and a spherical lens may be used as the collimator lens36.

[0033] A window hole 40A, through which the backward exiting light 34Rfrom the semiconductor laser 10 passes, and a window hole 40B, throughwhich forward exiting light 62 from the light wavelength convertingelement 14 passes, are formed in the package 38. The window hole 40A andthe window hole 40B are covered by a transparent window plate 42A and atransparent window plate 42B, respectively, such that the airtight stateis maintained. Further, a wire removal portion 44, which fits togetherwith a wire removal hole so as to keep a low melting point glass or thelike in an airtight state, is formed at the package 38. Two wires 46A,46B, which are bound to the electrodes of the semiconductor laser 10,pass through and are pulled-out from the wire removal portion 44.

[0034] The package 38, together with the mirror 12, is fixed on a baseplate 48 with the LD-SHG unit 20 and the collimator lens 36 sealed in anairtight state. An AR coating 50 is formed on the laser beam incidentside surface of the mirror 12. An HR coating 52 is formed on thesurface, of the mirror 12, which is at the side opposite the incidentside surface. A narrow-band bandpass filter 56 which serves as awavelength selecting element and which is held rotatably at a holder 54,a pair of total reflection prisms 58A and 58B for bending back theoptical path of the laser beam 34R by substantially 180°, a pair oftotal reflection prisms 58C and 58D for bending back by substantially180° the optical path which was bent back by substantially 180° by thepair of total reflection prisms 58A and 58B, and a collective lens 60for converging the laser beam 34R, which has been made into parallellight, on the surface of the HR coating 52 of the mirror 12, aredisposed in that order between the window plate 42A of the package 38and the mirror 12, and are fixed on the base plate 48. The HR coating 52of the mirror 12 preferably has a reflectance of 95% with respect to thefundamental wave.

[0035] The semiconductor laser 10 and the mirror 12 are disposed suchthat the resonator length (i.e., the optical length from the forwardexiting end surface of the semiconductor laser 10 to the surface of theHR coating 52 of the mirror 12) of the external resonator, which isformed by the mirror 12 and the forward exiting end surface of thesemiconductor laser 10, is longer than the coherent length of thefundamental wave exiting from the semiconductor laser. The coherentlength L of the fundamental wave is the intrinsic coherent distance ofthat laser beam, and can be calculated in accordance with the followingformula, where λ is the wavelength of the laser beam and Δλ is thespectral width.

L=λ ^(2/2)πnΔλ

[0036] Because the coherent length L of the fundamental wave isgenerally about 100 mm, the resonator length of the external resonatorcan be made to be a length which, for example, exceeds 100 mm.

[0037] Further, a collimator lens 64 which makes into parallel light asecond harmonic 62 (including a fundamental wave 34) which exits fromthe forward exiting end surface of the light wavelength convertingelement 14, an IR cutting filter 66 which removes the infrared lightcomponents from the second harmonic 62 (including the fundamental wave34) which has been made into parallel light, a half-wave plate 67 whichrotates by 90° the polarization direction of the second harmonic 62, ahalf-mirror 68, and a photodiode 70 are disposed at the outer side ofthe window plate 42B of the package 38, and are fixed on the base plate48. An aspherical lens which has little aberration is preferably used asthe collimator lens 64. Further, the half-mirror 68 and the photodiode70 are light-shielded, by a light shielding plate 73, from the opticalsystem forming the external resonator, such that scattered light is notincident on the photodiode 70.

[0038] The IR cutting filter 66 is disposed at an incline with respectto the optical axis. The half-wave plate 67 is disposed substantiallyorthogonal with respect to the optical axis, and preferably, so as toform an angle of no more than 0.5° with a plane which is orthogonal tothe optical axis. This is because, if the half-wave plate 67 is inclinedmore than 0.5° with respect to the optical axis, the extinction ratio ofthe light wavelength converting module deteriorates.

[0039] As shown in FIG. 2, the base plate 48 is fixed to a setting stand72. A Peltier element 74 is disposed between the base plate 48 and thesetting stand 72. Each of the optical elements fixed to the base plate48 are adjusted to predetermined temperatures by this Peltier element74. The respective optical elements fixed to the base plate 48 are,together with the base plate 48 and the Peltier element 74, covered by acover 75 for dust proofing whose laser beam exiting portion istransparent.

[0040] A knife edge 76, which serves as a light shielding plate for beamreshaping, is disposed and fixed, on the setting stand 72, in a vicinityof the position of convergence of the second harmonic 62. As will bedescribed later, the second harmonic 62, which exits after propagatingthrough the channel light waveguide 30 of the light wavelengthconverting element 14 in a first-order mode, has a side lobe at aportion which is beneath, in the vertical direction (the direction ofthickness of the substrate 26), the setting surface of the setting stand72. However, the knife edge 76 is disposed so as to cut this side lobeportion. A second harmonic 62G, which is obtained by the side lobe beingcut by the knife edge 76, is a Gaussian beam whose light intensitydistribution within the beam cross-section is a substantially Gaussiandistribution. Note that, in the present embodiment, the knife edge 76 isdisposed in a vicinity of the position of convergence of the secondharmonic 62. However, the knife edge 76 may be disposed so as to be fittightly to or to be adjacent to the forward exiting end surface of thelight wavelength converting element 14.

[0041] As shown in FIG. 3, the semiconductor laser 10 is connected to adriving circuit 78 via the wires 46A, 46B which are pulled-out to theexterior of the cover 75 for dustproofing. The schematic structure ofthe driving circuit 78 is shown in FIG. 4. The driving circuit 78 isformed from a DC power source circuit 80 having an automatic powercontrol mechanism (APC), an AC power source 84, and a bias T 88. Thebias T is formed from a coil 82 and a capacitor 86. A high frequencywave, which has been emitted from the AC power source 84 and has passedthrough the capacitor 86, is superimposed on the DC power sourcecomponent, which has been emitted from the DC power source circuit 80and has passed through the coil 82, and the current on which the highfrequency wave has been superimposed is applied to the semiconductorlaser 10. In order to reduce the noise of the second harmonic which isoutputted, the frequency of the high frequency wave which issuperimposed is preferably 300 to 400 MHz, and the degree of modulationis preferably 30 to 70%.

[0042] Two wires 71A, 71B are bonded to the electrodes of the photodiode70, and are pulled-out to the exterior of the cover 75 for dustproofing.The photodiode 70 is connected, via the wires 71A, 71B which have beenpulled-out to the exterior of the cover 75 for dustproofing, to the DCpower source circuit 80 which is equipped with the APC. The amount ofcurrent which is applied to the semiconductor laser 10 is controlled bythe APC such that the light output of the second harmonic 62 is apredetermined value. Further, the Peltier element 74 is connected to atemperature controller 90. A thermistor (not shown), for adjusting thetemperature within the device, is provided at the interior of the devicewhich is covered by the cover 75 for dustproofing. This thermistor aswell is connected to the temperature controller 90. On the basis of theoutput of the thermistor, the temperature controller 90 controls thePeltier element 74 such that the interior of the device is maintained ina temperature range at which the optical system does not freeze in theusage environment. (For example, if the usage environment temperature is30°, the temperature range at which the interior of the device ismaintained is 30° or more.)

[0043] Next, operation of the light wavelength converting module will beexplained.

[0044] The laser beam 34R (the backward exiting light), which is emittedfrom the semiconductor laser 10 toward the rear and not toward the lightwavelength converting element 14, is made into parallel light by thecollimator lens 36. The laser beam 34R which has been made into parallellight passes through the narrow-band bandpass filter 56. Thereafter, theoptical path is bent back 180° by the pair of total reflection prisms58A and 58B, and is again bent back by 180° by the other pair of totalreflection prisms 58C and 58D. The laser beam 34R is then collected bythe collective lens 60, and is converged on the mirror 12. The laserbeam 34R which is reflected at the mirror 12 follows back the opticalpath until then, and is fed-back to the semiconductor laser 10. Namely,in this device, the external resonator of the semiconductor laser 10 isformed by the mirror 12 and the forward end surface of the semiconductorlaser 10.

[0045] The wavelength of the laser beam 34R which is fed-back isselected by the narrow-band bandpass filter 56 which is disposed in theexternal resonator. The semiconductor laser 10 oscillates at theselected wavelength, and the selected wavelength changes in accordancewith the rotational position of the narrow-band bandpass filter 56.Thus, by appropriately rotating the narrow-band bandpass filter 56, theoscillation wavelength of the semiconductor laser 10 is selected to beand can be locked (fixed) at a wavelength which phase-matches the periodof the domain inverted portions 28 of the light wavelength convertingelement 14.

[0046] On the other hand, the laser beam 34, which is locked to apredetermined wavelength and has been emitted from the forward side ofthe semiconductor laser 10, enters into the channel light waveguide 30.The laser beam 34 is waveguided through the channel light waveguide 30in the TE mode, and is phase-matched (so-called pseudo phase matching)at the periodic domain inverted region thereof, and is converted intothe second harmonic 62 whose wavelength is ½ (e.g., when the centralwavelength of the laser beam 34 is 950 nm, the wavelength of the secondharmonic 62 is 475 nm). This second harmonic 62 also propagates throughthe channel light waveguide 30 in the guided wave mode, and exits fromthe light waveguide end surface.

[0047] From research conducted by the present inventors and others, ithas been learned that the overlapping integral, with the fundamentalwave, of a second harmonic which propagates through a light waveguide ina first-order mode, is greater than that of a second harmonic whichpropagates through a light waveguide in a zero-order mode. Namely, thewavelength converting efficiency is better when a fundamental wave and asecond harmonic which propagates in a first-order mode arephase-matched. Thus, in the present embodiment, the period Λ of theperiodic domain inverted structure is set such that the second harmonic62, which propagates through the channel light waveguide 30 of the lightwavelength converting element 14 in the first-order mode, and thefundamental wave 34 are pseudo phase matched. Specifically, given thatthe effective refractive index of the light waveguide with respect tothe fundamental wave is n_(ω), the effective refractive index of thelight waveguide with respect to the second harmonic is n_(2 ω), and thewavelength of the fundamental wave is λ_(F), the period Λ is set suchthat the following formula is satisfied.

n _(2 ω) − _(nω)=λ_(F)/2 Λ

[0048] Further, the laser beam 34 whose wavelength is not converted alsoexits from the light waveguide end surface in a state of being scatteredlight. The laser beam 34, together with the second harmonic 62, are madeinto parallel light by the collimator lens 64. After the light whichexits from the light waveguide end surface of the light wavelengthconverting element 14 is made into parallel light by the collimator lens64, the fundamental wave 34 is removed by the IR cutting filter 66 suchthat the second harmonic 62 is separated, and the polarization directionof the second harmonic 62 is rotated 90° by the half-wave plate 67, andthe second harmonic 62 exits. One portion of the second harmonic 62which has exited is reflected by the half-mirror 68 and detected by thephotodiode 70. Power control of the laser beam is carried out on thebasis of these results of detection.

[0049] As described above, in the light wavelength converting modulerelating to the present embodiment, the polarization direction of thesecond harmonic which exits from the light wavelength converting elementis the direction parallel to the setting stand. However, by using thehalf-wave plate for polarization control, a second harmonic which ispolarized in a direction orthogonal to the setting stand can beobtained. At this time, the half-wave plate is disposed between thelight wavelength converting element and the IR cutting filter. Thus, thefundamental wave is removed from the light which reaches the half-waveplate. Accordingly, the fundamental wave is not reflected by thehalf-wave plate and does not become return light, noise due to returnlight returning to the semiconductor laser is not generated, and a wavewhose wavelength is converted can be obtained stably.

[0050] Further, at the light wavelength converting module relating tothe present embodiment, the semiconductor laser and the light wavelengthconverting element are directly joined. Thus, with a simple structurewhich does not utilize a solid state laser crystal, the fundamental waveexiting from the semiconductor laser can be directlywavelength-converted by the light wavelength converting element. Thedegrees of freedom in selecting the oscillation wavelength increase, andhigh speed modulation can be carried out.

[0051] At the light wavelength converting module of the presentembodiment, because only a small number of parts, including thesemiconductor laser and the light wavelength converting element, aresealed airtight within the package, fabrication is easy. Moreover,because the number of parts which are sealed airtight is few,deterioration over time and the like of the parts which are sealed dueto the gasses generated from the respective parts can be prevented.

[0052] Further, the light wavelength converting module of the presentembodiment utilizes a mirror in which an AR coating is formed at thesurface at the laser beam incident side and an HR coating is formed atthe surface at the side opposite the incident side surface. Thus, thebeam spot diameter at the mirror surface becomes large, it is difficultfor dust and dirt to adhere to the mirror surface, and a deteriorationin the reflectance of the mirror due to the adhering of dust and dirtcan be prevented.

[0053] Moreover, in the light wavelength converting module relating tothe present embodiment, the laser light emitted from the semiconductorlaser is locked to a predetermined wavelength. Thus, a wave whosewavelength has been converted can be outputted stably. (Hereinafter, asemiconductor laser, at which the laser light emitted therefrom can belocked to a predetermined wavelength, is called a “wavelength stabilizedlaser”.)

[0054] Further, at the time of locking the wavelength, by making theresonator length of the external resonator longer than the coherentlength of the fundamental wave, interference due to return light iseliminated, and the linearity of the IL characteristic (the light outputcharacteristic with respect to the driving current) can be maintained.In a structure provided with an external resonator, lights of differentoptical path lengths, such as return light from the external resonator,are combined and become the exiting light. However, because lights ofdifferent optical path lengths interfere with one another, when thelight interfering state changes, there are cases in which the linearityof the IL characteristic deteriorates. For example, when the currentapplied to the semiconductor laser is increased, the semiconductor laseritself generates heat, and the refractive index and the length of thesemiconductor laser change. Thus, the oscillation wavelength of thesemiconductor laser changes. Such a change in the oscillation wavelengthchanges the light interfering state, and the linearity of the ILcharacteristic of a wavelength stabilized laser deteriorates. However,as in the present embodiment, when the resonator length of the externalresonator becomes longer than the coherent length of the fundamentalwave, even if the resonator length of the external resonator variessomewhat, there is no great effect on the oscillation wavelength of thesemiconductor laser, and the linearity of the IL characteristic of awavelength stabilized laser is improved.

[0055] Further, in the present embodiment, by sealing airtight a smallnumber of parts including the semiconductor laser and the lightwavelength converting element, changes in the humidity and theatmospheric pressure of the usage environment can be sufficientlyaddressed. Thus, in the light wavelength converting module of thepresent embodiment, a wave whose wavelength has been converted can bestably output. Note that, in the present embodiment, although theresonator length of the external resonator is long as described above,the light wavelength converting module is contrived to be made morecompact by the external resonator being made to be a structure in whichthe optical path is bent back.

[0056] Further, because the light wavelength converting module of thepresent embodiment uses a transverse single mode semiconductor laser,the problem of transverse mode hopping does not occur.

[0057] In the light wavelength converting module of the presentembodiment, the obtained second harmonic is a Gaussian beam. Thus, therecording light can be narrowed to a smaller spot, and can be suitablyused as the recording light source of a light scanning/recording device.

[0058] Further, in the light wavelength converting module of the presentembodiment, the semiconductor laser is modulated and driven by a highfrequency wave being superimposed on the driving current. Thus,longitudinal mode competition is suppressed. When the transmission bandof the wavelength selecting element is set to be wider than theFabry-Perot mode interval between both cleavage surfaces of thesemiconductor laser, the semiconductor laser oscillates in a pluralityof longitudinal modes. In this state, even if the driving current of thesemiconductor laser is fixed, a phenomenon known as longitudinal modecompetition, in which the rate of the power distribution to eachlongitudinal mode varies depending on the time, occurs. However, in thelight wavelength converting module of the present embodiment, thesemiconductor laser is modulated and driven by the high frequency wavebeing superimposed on the driving current. Thus, the driving currentdoes not accumulate at a region at which longitudinal mode competitionoccurs.

[0059] (Second Embodiment)

[0060] As shown in FIG. 5, a light wavelength converting module relatingto the present embodiment has the same structure as that of the lightwavelength converting module relating to the first embodiment, exceptthat a light attenuator 92, which serves as a light attenuatingmechanism, is provided between the IR cutting filter 66 and thehalf-wave plate 67. Thus, portions of the present embodiment which arethe same as those of the first embodiment are denoted by the samereference numerals, and description thereof is omitted.

[0061] As described above, the collimator lens 64 which makes intoparallel light the second harmonic 62 (including the fundamental wave34) which exits from the forward exiting end surface of the lightwavelength converting element 14, the IR cutting filter 66 which removesthe infrared light components from the second harmonic 62 (including thefundamental wave 34) which has been made into parallel light, the lightattenuator 92 which attenuates the incident second harmonic 62 to apredetermined light amount, the half-wave plate 67 which rotates by 90°the polarization direction of the second harmonic 62, the half-mirror68, and the photodiode 70 are disposed at the outer side of the windowplate 42B of the package 38, and are fixed on the base plate 48.

[0062] The light attenuator 92 is an element which attenuates, at apredetermined rate of attenuation, the amplitude (intensity) of thelight incident thereon. Light attenuators are classified into absorbingtypes which reduce the amount of light transmitted therethrough by lightabsorption, non-absorbing types which reduce the amount of lighttransmitted therethrough by a method other than light absorption,wavelength selection types and wavelength non-selection types whichattenuate the amplitude of incident light of a predetermined wavelength,fixed types which have a fixed rate attenuation, variable types whoserate of attenuation can be varied, and the like. Examples of a lightattenuator which is an absorbing type and a wavelength non-selectiontype are ND filters and the like. An example of a light attenuator whichis a non-absorbing type and a wavelength selection type is a dielectricmultilayer film. An example of a light attenuator which is a wavelengthnon-selection type and a variable type is a combination of twopolarizers which can change the direction of the axis of transmission.In the present embodiment, an ND filter is used, but another type oflight attenuator may be used.

[0063] After the light which exits from the light waveguide end surfaceof the light wavelength converting element 14 is made into parallellight by the collimator lens 64, the fundamental wave 34 is removed bythe IR cutting filter 66 such that the second harmonic 62 is separated.The second harmonic 62 is attenuated by the light attenuator 92 to apredetermined light amount, and is made incident on the half-wave plate67. The polarization direction of the second harmonic 62 is rotated 90°by the half-wave plate 67, and the second harmonic 62 exits. One portionof the second harmonic 62 which has exited is reflected by thehalf-mirror 68 and detected by the photodiode 70. Power control of thelaser beam is carried out on the basis of these results of detection.

[0064] Here, the role of the light attenuator 92 will be described infurther detail. The IL characteristic of a wavelength stabilized laseris shown in FIG. 6. As shown by the solid line in FIG. 6, when thedriving current falls within a predetermined range, the light output ofthe wavelength stabilized laser is stable. However, for example, when anattempt is made to obtain a desired output light amount Ld, the drivingcurrent falls outside of the predetermined range, and dispersion arisesin the light output of the wavelength stabilized laser. At this time, asshown by the dashed line in FIG. 6, by attenuating the output light ofthe wavelength stabilized laser at a predetermined ratio, the desiredoutput light amount Ld can be stably obtained at a driving current whichfalls within the predetermined range.

[0065] In accordance with the light wavelength converting modulerelating to the present embodiment, the same effects as those of thefirst embodiment can be obtained. In addition, by providing the lightattenuator, which attenuates the light transmitted therethrough, betweenthe IR cutting filter and the half-wave plate, the transmitted light canbe attenuated in accordance with a desired output light amount such thatthe value of the driving current of the semiconductor laser falls withina predetermined range, and a stable output can be obtained. Further, inthe present embodiment, the second harmonic which is reflected by thehalf-mirror is detected by the photodiode and monitored. By providingthe light attenuator at one place at the light exiting side of thehalf-mirror, dispersion in the value of the monitor current can besuppressed.

[0066] In the above-described second embodiment, although an example isdescribed in which the light attenuator is provided between the IRcutting filter and the half-wave plate, the light attenuator may bedisposed at any position at the light exiting side of the lightwavelength converting element. However, in a case in which thecollimator lens 64, the IR cutting filter 66, the half-wave plate 67,and the half-mirror 68 are disposed in that order at the light exitingside of the wavelength converting element 14 as shown in FIG. 1, it ispreferable to provide the light attenuator at the light exiting side ofthe collimator lens 64, from the standpoint of preventing return lightdue to reflection from the light attenuator. In a case in which an NDfilter is used as the light attenuator, the problem of return light dueto reflection does not arise, and thus, the light attenuator may beprovided between the wavelength converting element 14 and the collimatorlens 64. Further, as shown in FIG. 1, in the case of a structure inwhich a portion of the exited second harmonic 62 is reflected at thehalf-mirror 68 and detected by the photodiode 70, it is preferable toprovide the light attenuator at the light incident side of thehalf-mirror 68. When the light attenuator is provided at the lightincident side of the half-mirror 68, light attenuators must be providedat two places.

[0067] In the above-described second embodiment, an example is describedin which a light attenuator is provided as the light attenuatingmechanism. However, it suffices to not use a light attenuator, andinstead, to provide a light attenuating mechanism by forming a coatingfor adjusting the light transmittance on another part which is disposedat the light exiting side of the light wavelength converting element.For example, in the light wavelength converting module shown in FIG. 1,it is possible to form a coating for adjusting the light transmittance,at at least one of the collimator lens 64, the IR cutting filter 66, thehalf-wave plate 67 and the half-mirror 68.

[0068] In the above-described first and second embodiments, an exampleis described in which total reflection prisms are used to bend back theoptical path of the external resonator. However, instead of totalreflection prisms, total reflection mirrors may be used. In this case,the total reflection mirrors are disposed such that the reflectingsurfaces thereof correspond to the positions of the inclined surfaces ofthe total reflection prisms.

[0069] In the above-described first and second embodiments, an exampleis described in which a half-wave plate is used as the wavelength plate.However, the type of the wavelength plate can be appropriately changedin accordance with the purpose of use of the light wavelength convertingmodule. For example, in a case in which it is desired to obtaincircularly polarized light as the output light of the light wavelengthconverting module, a quarter-wave plate may be used in place of thehalf-wave plate. Note that, other than a half-wave plate and aquarter-wave plate, wavelength plates having various phase differences,such as an eighth-wave plate, a three-quarter-wave plate, and the like,can be obtained.

[0070] In the above-described first and second embodiments, an exampleis described in which the semiconductor laser and the light wavelengthconverting element are joined directly. However, the semiconductor laserand the light wavelength converting element may be joined via a lens.

[0071] In the above-described first and second embodiments, wavelengthlocking can be mitigated and the linearity of the IL characteristic canbe improved by finely adjusting the positions at which the membersforming the external resonator are disposed or the positions at whichthe members disposed within the external resonator are disposed, so asto reduce the return light to the semiconductor laser due to theexternal resonator. To describe this concretely with reference to FIG.1, for example, by setting the collimator lens 36 slightly closer towardthe semiconductor laser 10 such that the spread angle θ of thecollimator lens falls within the range 0 °<θ<30°, the return light tothe semiconductor laser 10 is decreased, and the linearity of the ILcharacteristic is improved. This depends on the performances of the endportion of the collective lens 60 being poor, vignetting occurring dueto spreading at the collective lens 60, and the like. Further, forexample, by setting the mirror 12 slightly closer to the semiconductorlaser 10 such that the position at which the mirror 12 is disposed isoffset from the focal point position of the collective lens 60, thereturn light to the semiconductor laser 10 is decreased, and thelinearity of the IL characteristic is improved.

[0072] In accordance with the present invention, the light wavelengthconverting module, which is formed to include a light wavelengthconverting element and a semiconductor laser which is optically coupledto the light wavelength converting element, has the effect that, when awavelength plate, which provides a predetermined optical pathdifference, is provided between two orthogonal polarized lightcomponents of a wave whose wavelength has been converted, the generationof noise due to return light is prevented, and a wave whose wavelengthhas been converted can be obtained stably.

What is claimed is:
 1. A light wavelength converting module comprising:a semiconductor laser from which a fundamental wave exits; a lightwavelength converting element which is optically coupled to thesemiconductor laser, and which converts a wavelength of the fundamentalwave which has entered from the semiconductor laser; a wavelength platedisposed at a light exiting side of the light wavelength convertingelement; and a removing portion, disposed between the wavelength plateand the light wavelength converting element, for removing thefundamental wave from light incident on the removing portion.
 2. A lightwavelength converting module according to claim 1, wherein the removingportion is an IR cutting filter.
 3. A light wavelength converting moduleaccording to claim 1, wherein the light wavelength converting element isdirectly joined to the semiconductor laser.
 4. A light wavelengthconverting module according to claim 1, wherein the wavelength plate isone of a half-wave plate and a quarter-wave plate, with respect to awave whose wavelength is converted.
 5. A light wavelength convertingmodule according to claim 1, wherein the wavelength plate is disposedsubstantially orthogonal to an optical axis.
 6. A light wavelengthconverting module according to claim 1, wherein a beam splitter isprovided at a light exiting side of the wavelength plate.
 7. A lightwavelength converting module according to claim 1, wherein a beamsplitter and a photodiode are disposed at a light exiting side of thewavelength plate, and the beam splitter and the photodiode are shieldedfrom light.
 8. A light wavelength converting module according to claim1, wherein a light attenuating portion, which attenuates light passingtherethrough, is provided at a light exiting side of the lightwavelength converting element.
 9. A light wavelength converting moduleaccording to claim 6, wherein a light attenuating portion, whichattenuates light passing therethrough, is provided at a light exitingside of the wavelength converting element and at a light entering sideof the beam splitter.
 10. A light wavelength converting module accordingto claim 7, wherein a light attenuating portion, which attenuates lightpassing therethrough, is provided at a light exiting side of thewavelength converting element and at a light entering side of the beamsplitter.